1. Field of the Invention
The present invention relates to the field of optoelectronic devices and more particularly to a waveguide-based photodetector device and manufacturing method thereof.
2. Discussion of Related Art
Over recent years, the increasing computing power of integrated circuits, such as a microprocessor, has been enabled by downscaling the size of basic devices such as transistors and metal interconnects. However, the smaller dimensions of metal interconnect lead to undesirable effects such as increased interconnect delays, electromagnetic interference (EMI) and power consumption. In that regard, optical interconnects have been proposed as an alternative to metal interconnects. Optical interconnects have the advantages of being resistant to EMI and decrease interconnect delays and power consumption. Furthermore, the optical interconnect serves as a better alternative in keeping pace with increasing transistor speed as it offers higher bandwidths than metal interconnects.
One type of optical interconnect involves the use of a waveguide-based photodetector device comprising a photodetector element coupled to an optical waveguide. The optical waveguide serves as a medium for guiding light containing an optical signal to the photodetector element which is able to detect the optical signal and converts it into an electrical signal. In a conventional waveguide-based photodetector device, the photodetector element is evanescently coupled to the optical waveguide such that the photodetector element is disposed either underneath or on top the optical waveguide with only one common interface.
FIG. 1 illustrates a cross-sectional view of a conventional waveguide-based photodetector device comprising a substrate 110, an oxide layer 120 and a rib waveguide 130 for receiving light. The photodetector device is made from a silicon on insulator (SOI) substrate such that the rib waveguide 130 is patterned from the top silicon film of the SOI substrate. A photodetector element 140 made of germanium (Ge) is disposed on the top surface 135 of the rib waveguide 130 to absorb light from the rib waveguide 130 and generate an electrical signal based on the amount of light absorbed. N-type contact 161 and p-type contact 162 are coupled to the photodetector element 140 to transmit the electrical signal to other devices.
As illustrated in FIG. 1, the photodetector element 140 is evanescently coupled to the rib waveguide 130 such that photodetector element 140 is disposed on the top surface 135 of the rib waveguide 130. In this case, the length of the photodetector element 140 has to be at least 40 μm long in order to obtain an effective coupling efficiency. A weak coupling efficiency between the photodetector element 140 and the rib waveguide 130 affects the speed and responsivity of the photodetector device. In addition, the 40 μm long photodetector element 140 takes up a considerable amount of “real estate” in the device layer of an integrated circuit based on current technology standards. Furthermore, the photodetector device is made from part of a SOI substrate, which makes it difficult to integrate the photodetector device into later stages of an integrated circuit fabrication process.